Due to its lack of moving parts and potential for conformal installation on vehicles, missiles and aircraft, the electrically scannable (E-scan) antenna is an important weapon in the arsenal of the Army's Future Combat Systems.
Most approaches to designing such antennas involve some type of phased array, in which the antenna beam is created by superimposing the outputs of many antenna subelements. Steering this beam is implemented by phase-shifting the input signals to these antenna elements relative to one another via phase shifters. The use of these phases shifters for beam steering has made it imperative that a simple, low-cost method be identified for electrically controlling them. However, existing approaches to designing phased-array antennas involves complicated sub-circuits with mixers, amplifiers, and the like, feeding each antenna element. Such circuitry makes the radius complicated, unreliable, and expensive to manufacture and maintain.
Recent work at the Army Research Lab on E-scan antennas as part of the Multifunction RF STO has centered on two architectures for such passed arrays: one based on the use of hundreds of discrete phase shifters, one for each antenna subelement, and the other a “true-time delay” approach in which a single tapped delay line is used to generate and phase all the signals sent to the antenna array elements at the same time.
In the true time-delay approach, a time-dependent input signal is launched as a wave on a waveguide. Electrodes (“taps”) placed along the waveguide at equal intervals generate replicas of this input signal that are delayed relative to one another by the time the wave takes to go from one tap to another. In contrast to the discrete phase shifter approach, with its hundreds of elements, this approach makes possible the simultaneous generation of as many signals as are needed from a single monolithic element, the waveguide. When used in this fashion, the waveguide is referred to as a “delay line”.
Some delay lines have the property that the delays imposed on the signal replicas appearing at its taps are the same regardless of the underlying signal frequency (in the art such a line is said to be “non-dispersive”). When this is true, even complex time-dependent signals consisting of many frequencies (so-called “broadband” signals) can be used to steer antenna beams in one direction without drifting or unintentional scanning. In contrast, the discrete phase shifter approach restricts the complexity of input signals lest they interfere with the steering in the specified direction, which makes them useless for sophisticated radar applications.
In order to electrically steer the antenna, it is necessary to electrically control the phase shifts imposed on the signal replicas sent to the antenna elements. It has long been known that electrically controllable phase shifters can be made by using ferroelectric materials, by virtue of the nonlinear dielectric response of the latter. Combining this choice of materials with the true-time delay approach leads to the novel concept of an electrically controllable delay line. Such a line can support the propagation of a signal along it like any other delay line, and can be tapped in the same way, leading to phase shifts between the taps. The choice of dielectric determines how much delay is obtained per length of line.
However, if the dielectric used to make the line is also a ferroelectric, the line properties can be changed by “biasing” it with a DC voltage. The simplest way to implement such a line is to make it a microstrip, consisting of a ferroelectric layer on top of a metal ground plane with a narrow strip of metal on top of the ferroelectric layer. The input signal propagates along this metal strip as a voltage between the top conductor and the ground plane. This type of line is non-dispersive as defined above, so that complex signals can be used with it. In addition to this signal, a DC bias can be applied in the same way. Because the bias changes the RF propagation velocity, the delay, and hence the phase shift, can be controlled by the bias. This control applies to all the multiple versions of the signal obtained from the taps, i.e., all the phase shifts are controlled by a single DC bias. In principle, one delay line could steer an entire antenna array.
Unfortunately, such use of ferroelectrics is not without problems. Because dielectric constants are extremely high in these materials, the wavelengths of electromagnetic waves that propagate in them are very short, which leads to “too much” phase shift per centimeter of line. In addition, the loss per centimeter down the line is extremely high.
It can be shows that in order for a phased array antenna fed by a delay line to generate a strong main beam, the distance between delay line taps D must satisfy the relation
                    D        d            ⁢              ɛ              <    1    ,where d is the spacing between antenna array subelements. Because d is commonly chosen to be λ/2, where λ is the free-space wavelength of the radar signal and is typically a few centimeters down to a millimeter for military applications, working with a ferroelectric in which ε is, e.g., 1000 requires values of D<d/30, i.e., the delay line taps must be extremely close together.
These parameters make a microwave-based delay line using ferroelectrics difficult to manufacture. In addition, the dielectric constant of a pure ferroelectric material is extremely sensitive to temperature, and typically is lossy as well, which may distort the shape of the antenna beam and produce unintended beam motion.